Stronger demand has recently arisen for higher integration and micropatterning for a variety of devices.
Finer patterns than the conventional ones must be formed for semiconductor integrated circuits. X-ray proximity exposure techniques which use as exposure light X-rays having a shorter wavelength than that of conventional exposure light have received a great deal of attention in photolithography.
Proximity X-ray lithography has conventionally been known in which a mask pattern is transferred to a wafer by exposure, disposed near the mask, in a one-to-one size using as a light source X-rays having a wavelength of 7 to 10 Å (0.7 to 1 nm) emitted from an electron storage ring (to be referred to as an SR hereinafter) serving as a synchrotron radiation apparatus.
FIG. 9 is a view showing the arrangement of an X-ray exposure apparatus disclosed in, e.g., NTT R&D, Vol. 43, P. 501 (1994).
The X-ray exposure apparatus shown in FIG. 9 is comprised of a synchrotron radiation source 101, an X-ray mirror 103, a heat removal filter 104, a beryllium window 105, a window 122 formed from a silicon nitrite film, an X-ray mask 106, and a vertical X-Y stage 123 on which a semiconductor wafer 109 is placed. Light or synchrotron radiation 102 emitted from the synchrotron radiation source 101 passes through the X-ray mirror 103, heat removal filter 104, beryllium window 105, and window 122 formed from the silicon nitride film and reaches the X-ray mask 106. A circuit pattern to be transferred to the semiconductor wafer 109 is formed on the X-ray mask 106 from an X-ray absorber. The light 102 passes through the X-ray mask 106 to transfer the circuit pattern onto the resist applied to the semiconductor wafer 109.
The light 102 is continuous spectral light having wavelengths in the wide range from the X-ray range to the infrared range. X-rays required in the X-ray exposure process for transferring the pattern onto the semiconductor wafer 109 must have an appropriate wavelength range. For this reason, in a conventional X-ray exposure apparatus, X-ray components having wavelengths of about 0.7 nm or less are absorbed and cut using the reflection characteristics of the X-ray mirror 103. When the light 102 passes through the heat removal filter 104, most of the X-ray components of 1.5 nm or longer are absorbed and cut by the heat removal filter 104 in accordance with the properties of the beryllium material.
The wavelength of the light 102 is adjusted to fall within the range of about 0.7 to 1.5 nm. The light sequentially passes through the beryllium window 105 and window 122 formed from the silicon nitride film. In this case, almost no heat is generated by the beryllium window 105 and the window 122 formed from the silicon nitride film.
The space between the beryllium window 105 and the window 122 formed from the silicon nitride film is filled with atmospheric helium. The beryllium window 105 serves as a partition wall between the vacuum area upstream of the beryllium window 105 and the atmospheric pressure area downstream of the beryllium window 105. Unwanted X-ray components are cut by the heat removal filter 104 to suppress heat generation of the beryllium window 105. As a result, this keeps the mechanical strength of the beryllium window 105 high.
The window 122 formed from the silicon nitride film serves as a partition wall between the helium layer and outer air. When the vertical X-Y stage 123 is disposed in the helium atmosphere, the window 122 formed from the silicon nitride film can be omitted.
As described above, the circuit pattern to be transferred to the semiconductor wafer is formed on the X-ray mask 106. The predetermined area of the resist applied to the semiconductor wafer 109 is irradiated with the light 102 through the X-ray mask 106, thereby transferring this circuit pattern onto the semiconductor wafer 109.
The surface of a heavy metal such as gold or platinum has been used as the conventional material of an oblique incident mirror (X-ray mirror). This is because a reflectance of about 60% can be obtained at an exposure wavelength of 0.7 nm even if the oblique incident angle of the light 102 is set as large as about 2°. The viability of converging a larger amount of X-rays by forming an X-ray mirror having a larger convergence angle using a material such as gold or platinum has been examined.
The convergence of a larger amount of X-rays allows increasing the intensity of X-rays used in exposure. As a result, a high throughput can be obtained in the exposure process.
The use of silicon carbide or molten quartz has been proposed as the material of the X-ray mirror 103. Silicon carbide makes it possible to increase the X-ray reflectance as very high as about 90% at an X-ray wavelength of 0.7 nm or more by setting the oblique incident angle as relatively small as 1°.
A beryllium thin film is proposed as the material of the heat removal filter 104 for absorbing and cutting X-rays having longer wavelengths (wavelengths: 1.5 nm or more). A silicon nitride or diamond thin film is also proposed as an additional thin film. This thin film aims at increasing the heat absorption efficiency and preventing oxidation of the beryllium thin film.
The X-ray mask 106 is comprised of a membrane generally made of silicon carbide and an X-ray absorber formed on the membrane. Silicon carbide is used because it has a relatively small absorbance of exposure X-rays having wavelengths of about 0.7 to 1.5 nm.
As the material of an X-ray mirror surface for reflecting X-rays, gold, platinum, silicon carbide, and molten quartz are proposed. As the window material, beryllium, silicon nitride, and diamond are proposed. For any of these materials, it is assumed that X-rays having a peak wavelength of about 0.75 nm, which have been said to be optimal for X-ray exposure, are used as exposure light. The reason why X-rays having the peak wavelength of about 0.75 nm are suitable as optimal exposure light is as follows.
In principle, the shorter the X-ray wavelength, the higher the resolution of an optical image. This makes it possible to form a finer pattern. However, the shorter the X-ray wavelength, the higher the X-ray energy. When the resist applied to the semiconductor wafer 109 is irradiated with X-rays during the exposure process, photoelectrons are generated in the resist. The kinetic energy of the photoelectrons increases with an increase in incident X-ray energy.
The resist is photosensitized with the photoelectrons. When the wavelength of the X-rays becomes shorter, the resist area photosensitized with the photoelectrons generated in the resist increases. A pattern formed in the resist blurs due to the influence of the photoelectrons. That is, the range of photoelectrons determines the resolution limit.
The optimal peak wavelength of X-rays used in exposure has conventionally been said to be about 0.75 nm.
As described above, the range of photoelectrons is assumed to determine the resolution limit. In the conventional exposure process using X-rays having a peak wavelength of about 0.75 nm described above, it has been said that a pattern having a line width or line spacing of 100 nm (0.1 μm) or less cannot be formed.
Under these circumstances, in order to increase the resolutions in the X-ray exposure processes, proposals for higher resolutions have been made by using a low-contrast mask, a phase shift mask whose absorber pattern is tapered in the vertical direction, and a mask whose optical proximity effect is corrected. In any case, it is difficult to greatly increase the resolution.
A technique for shifting the wavelength of X-rays used for exposure to a shorter wavelength range and transferring a circuit pattern to obtain a higher resolution has not conventionally been examined in the technical field of semiconductor manufacturing apparatuses due to the above-mentioned problem of the range of photoelectrons. In addition, the X-rays in the shorter wavelength range are used, the X-ray energy is higher than the conventional one, so the X-rays are readily transmitted through the absorber of an X-ray mask.
To obtain necessary contrast, it is assumed that the thickness of an X-ray absorber must be increased. When X-rays pass through a transfer pattern made of an X-ray absorber having such a large thickness, the transmission characteristics of X-rays degrade due to the waveguide effect to result in a decrease in resolution of the circuit pattern to be transferred. It has conventionally been difficult to downsize the transfer pattern by shortening the X-ray wavelength.
As an exposure technique using short-wavelength X-rays, an example using an exposure wavelength of about 0.3 nm is available in the field of micromachine techniques. The use of short-wavelength X-rays aims at high-aspect patterning in which a pattern of several μm is formed at a height of several hundred μm by increasing the transmission of X-rays in the resist. The pattern size required in the field of micromachine techniques is larger than that in the field of semiconductor manufacturing apparatuses by one or two orders of magnitude.
The thickness of an X-ray absorber in an X-ray mask used in the field of micromachine techniques is larger than that in the field of semiconductor manufacturing apparatuses. In addition, an X-ray mask substrate is made of a metal such as titanium. The above technique is not to transfer an ultrafine circuit pattern and belongs to a technical field entirely different from that of the present invention. As another conventional example, an experimental sample has been reported in which a mask obtained by forming an absorber on a boron nitride substrate with gold plating is used in an exposure apparatus using an electron excitation type point source using a palladium target to perform exposure using X-rays having a wavelength range of 0.415 nm to 0.44 nm. This technique also belongs to a technical field basically different from that of the present invention using a synchrotron radiation source.
In a conventional X-ray exposure apparatus and method for forming a pattern using X-ray proximity exposure techniques, demand has arisen for widening a pattern application limit to a finer pattern range and transferring a high-resolution fine pattern onto a substrate at high speed. As problems in short-wavelength exposure in the X-ray proximity exposure techniques, fogging occurs in a resist or on a substrate due to photoelectrons and Auger electrons due to exposure light. As a result, the pattern resolution decreases to make it impossible to form finer patterns.
That is, the resolution limit is determined by the range of photoelectrons generated in the resist by exposure light, and a system having a configuration optimized on the basis of the above assumption has been employed.
In recent years, resists having a dissolution rate ratio of a resist portion influenced by photoelectrons and a resist portion directly irradiated with X-rays have been developed. In addition, necessity of micropatterns having sizes almost reaching 0.05 μm has become evident.